conical intersection
Point of crossing between two electronic states of the same spin multiplicity (most commonly singlets or triplets).
  1. In a polyatomic molecule two @P04780@ are allowed to cross along a (3N − 8)-dimensional subspace of the (3N − 6)-dimensional nuclear coordinate space (the @IT07400@) even if they have the same spatial/spin symmetry (N is the number of nuclei). Each point of the @IT07400@ corresponds to a conical intersection. If the energy is plotted against two special internal geometrical coordinates, x1 and x2, which define the so-called @BT07335@, the potential energy surface would have the form of a double cone in the region surrounding the @D01556@. In the remaining (3N − 8) directions, the energies of the @G02704@ and @E02257@ remain degenerate; movement in the @BT07335@ lifts the @D01556@.
  2. From a mechanistic point of view, conical intersections often provide the @C00970@ mediating @R05056@ and @P04585@.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 317 (